Fractures and bone pain constitute problems for elderly women. Bone loss is a major factor in these problems, but the mechanism is not clear. Postmenopausal osteoporosis may be a concomitant of normal aging or a disease affecting only a fraction of the aging population. Cross sectional studies of relative skeletal status and longitudinal studies of fracture incidence have not clarified the situation. We will investigate this problem by repeated bone mineral determination by monoenergetic photon absorptiometry and measurement of bone resonant frequency of 200 normal women over a period of 2 years. The women will range from 35 to 55 years of age at the beginning of the study and will receive physical examinations and blood and urine chemistries to rule out conditions such as hyperparathyroidism, rheumatoid arthritis, etc, which may have generalized skeletal manifestations. This study will show individual rates of bone loss vs. age, height, weight, parity, etc.; whether a distinct subpopulation with a relatively rapid rate of bone loss exists; whether changes in resonant frequency precede changes in mineral (suggesting an alteration of bone quality), or whether the 2 changes occur simultaneously (suggesting simple changes in bone mass); and normative values of skeletal condition in women.